Although colored lead pencils have provided enjoyment to children and others for a long time, attempts have been made to improve upon the properties of these pencils. The properties that make coloring a pleasant and rewarding experience include smooth laydown of the lead composition, good color intensity of the markings, good break strength, and sufficient lead bending strength. In addition, it is desirable to have colored lead pencils that can produce markings on paper that do not smudge or smear and that are erasable by ordinary pencil erasers.
Common colored pencil leads are made from a blend of a pigment, a binder resin and a filler. Although these colored leads possess excellent writing qualities, their markings cannot be easily erased by ordinary pencil eraser. Attempts have been made to improve the lead properties, particularly the erasability of the lead markings. For example, attempts have been made to improve erasability by educing the pigment content of pencil leads. Although his approach reduces the color contrast between the erased ark and the paper, the color intensity of the original ark has been unsatisfactory.
In another approach to improve erasability of the markings, colored pencil leads have been made by blending a mixture of a pigment, one or more fillers, and a wax such as a low melting wax. Although this approach has improved the erasability of the marking, complete removal of the marking has not been achieved. In addition, the erased region is left with a smeared appearance. It is believed that the wax, which on the one hand helps erasability, on the other hand, also contributes to the incompleteness of erasure and the smearing.
It is believed that the shear forces generated during the act of marking on paper with a colored pencil are sufficient to cause a warming of the wax. This warming provides a degree of fluidity to the wax, and to the lead composition, thereby allowing significant penetration of the pencil lead material into the open spaces between the paper fibers. This results in a significant amount of colored pencil lead material becoming trapped below the surface of the paper. The pencil lead material which has so penetrated into the paper fiber matrix becomes unavailable for abrasion and removal by an eraser.
Additionally, the act of erasure provides further frictional forces which also warm the wax component of the colored pencil lead marks. The act of erasure, therefore, gives a degree of fluidity to the pencil mark already on the paper surface. This leads to further penetration by wax into the open spaces in the paper matrix, thereby making a further amount of the pigment contained within the pencil mark unavailable for mechanical abrasion by an eraser.
Further, the fluid nature of the wax results in a large portion of the pencil mark being moved, in the act of erasure, from one location to another without removal from the paper surface. This relocation of the material is generally known as smearing. Repeated use of an eraser on the colored pencil mark continues to smear and drive the pencil mark into the paper fiber matrix until substantially all of the available colored pencil material has been deposited in the open spaces of the paper matrix. At such a point, further attempts at erasure produce substantially no change in the colored pencil mark.
Attempts have been made to prepare colored lead pencils to eliminate the smearing and erasure residue. For example, in one approach, the wax content of the lead has been reduced. Unfortunately, this approach makes the lead very hard. The laydown characteristics of the lead becomes affected and the lead does not write well. In yet another approach, the use of a low melting substance such as an oil has been attempted. Colored pencil leads have been made by blending a pigment, a binder resin, a filler, and an oil. This approach is not easily practicable in commerce because either it increases the manufacturing cost or it affects certain other qualities of the lead. For example, if the lead is produced in a wet forming process, the wet lead should be dried at a temperature that is lower than the melting or flowing temperature of the oil. This increases the lead dry time, and consequently, reduces manufacturing productivity. The oil also tends to seep or migrate out of the lead. This could lead to loss of lead strength as well as failure to produce a mark of appropriate color strength. Further, the oil tends to bleed through the paper, thereby further affecting the quality of the coloring. To alleviate some of the drawbacks associated with the use of an oil, complex manufacturing processes have been proposed which unfortunately can increase the number of manufacturing steps and the pencil manufacturing cost. Common colored pencil leads containing a low melting wax or oil produce marks which tend to smudge when contacted with objects such as finger, paper, and blending stump.
Thus, there exists a need for a colored pencil lead having improved erasability. There further exists a need for colored pencil leads which do not smear when erased by an ordinary pencil eraser. There further exists a need for colored pencil leads which do not smudge when rubbed with a finger, blending stump, or other objects. There further exists a need for a colored pencil lead that has smooth laydown, good color strength, adequate bending strength, and adequate break strength.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.